(1) Field of the Invention
The present invention relates to the field of nuclear medicine systems. Specifically, the present invention relates to signal processing systems for gamma camera detectors.
(2) Prior Art
Scintillation detectors of gamma cameras are composed of an array of photomultipliers (PMTs) each generating a channel signal in response to gamma events detected within a crystal layer that is optically coupled to the PMT array. Gamma rays that radiate the crystal layer create scintillations releasing light energy that is detected by the PMT array. A collimator made of lead is typically placed in front of the crystal layer in order to collimate gamma rays entering the scintillation detector. The collimator is composed of a ring or peripheral region of solid lead with an interior region of small lead tubes arranged in a honeycomb matrix. When installed within the scintillation detector, the collimator's solid lead edge region (e.g., first region) typically cover portions of the photomultipliers located along the periphery of the PMT array. These photomultipliers are partially or totally obscured from receiving light energy from scintillations occurring directly above their surfaces. Therefore, in the past, it was necessary to remove the collimator when performing calibration of the PMT array in order to effectively calibrate the peripheral PMTs.
When the scintillation detector is assembled, the photomultipliers are originally (initially) calibrated so that each have the same effective gain. The effective gain of a photomultiplier is composed of two values. The first value is the gain associated with the PMT itself, Gt, which is a result of the physical characteristics of the tube. The second component of the effective gain is the gain applied to the output channel signal for each PMT, Gp. This gain, Gp, is applied via preamplification circuits coupled to receive channel signal outputs originating from the PMTs of the PMT array. Together the effective gain of a photomultiplier is Gt * Gp and should remain fixed for proper operation of the gamma camera systems.
The characteristic gain, Gt, of a particular PMT may deviate over time due to several conditions, some of which are discussed herein. This causes the effective gains of the individual PMTs of the PMT array to vary independently over time. The characteristic gain, Gt, of a PMT may vary due to release of internal gases of the tube or due to erosion the photocathode of the PMT. Also, cracks within the PMT may cause leakage or a decrease in the vacuum of the tube which also will cause deviations in the characteristic gain due to ion interference within the multiplication process of the tube. Therefore, what is needed is a system for providing compensation or calibration to correct for the deviations in the characteristic gain of a particular PMT. The present invention offers such advantageous capability.
As is well known, uniformity correction tables or factors are provided within the signal processing and image generation circuitry of gamma cameras to correct for non-uniformities inherent in the scintillation detectors. These uniformity correction tables are generated within the gamma camera system typically at the manufacturing site (after initial calibration of the scintillation detector is complete) and can be generated empirically based on the characteristics of a particular scintillation detector. The correction factors are typically generated or created based on calibrated PMTs of the PMT array when the array is initially calibrated such that the effective gains of each PMT are uniform. Therefore, over time as the characteristic gains of the PMTs change, the responses of the scintillation detector will change with reference to the uniformity correction factors which stay constant over time. What is needed is a system that can compensate for this deviation so that the scintillation detector provides the same response (over time) as it did when it was initially calibrated and when the uniformity correction factors were generated. The present invention provides such advantageous capabilities.
In the prior art, the collimators are removed to perform gain calibration on the scintillation detector. Since the collimators used in gamma camera technology are relatively heavy (e.g., 500 lbs) and very awkward to handle, it is not advantageous to remove the collimators on a routine basis from the scintillation detectors. Therefore, what is needed is an automatic gain calibration system that can automatically adjust the preamplification gains of a PMT array while the collimator is allowed to remain installed on the scintillation detector. What is further needed is a system as above that allows calibration of the peripheral PMT whose center surfaces may be partially covered or totally covered by the solid lead edge regions of the collimator. The present invention allows such advantageous results.
Accordingly, it is an object of the present invention to improve signal processing within a gamma camera system. It is an object of the present invention to improve the gain stability of photomultipliers within a scintillation camera system. It is another object of the present invention to provide an automatic gain calibration system applicable for all PMTs of a scintillation detector while the collimator remains installed on the detector. It is further an object of the present invention to maintain the original calibration of the PMT arrays so that the effective gains of the PMTs substantially match the original detector response used in creation of uniformity correction factors. These and other objects of the present invention not specifically mentioned above will become clear within discussions of the present invention herein.